Ghost Illusions Hide Objects in Plain Sight

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Ghostly illusions could one day help disguise military aircraft
for greater stealth, researchers say.

In the last eight years or so, scientists have discovered
cloaking devices are possible, which can bend and twist light
completely around objects, rendering them invisible. Cloaking
devices that work against other kinds of waves are possible as
well, such as the
acoustic waves used in sonar.

However, such cloaks are usually limited to working against
narrow ranges of frequencies for various types of waves. An
international team of physicists instead explored devices that
could potentially work against wide bands of frequencies,
generating illusory
ghosts as disguises.

"Our work has enormous potential to enhance our ability to mold,
harness and perceive waves at will," said researcher Cheng-Wei
Qiu, a physicist at the National University of Singapore.

The researchers created their prototype ghost illusion device
from eight concentric rings of flexible plastic circuit boards,
each a quarter-millimeter thick and about 3 millimeters (one-10th
of an inch) apart. They coated these sheets with rectangular
loops of copper about 3 millimeters wide on each side. This
coating of copper was only 35 microns thick, about a third the
average diameter of a human hair.

In experiments, the scientists placed a small metallic cylinder
within the ghost illusion device. The rings scattered incoming
waves from a radar scan, camouflaging the metal object as three
items — one shrunken object in the original position and two
ghost images on each side. [ Eye
Tricks: Gallery of Optical Illusions ]

"We are now working with some defense agencies to consider a
large-scale project," Qiu told LiveScience.

In principle, the researchers can manipulate what illusions are
seen by tinkering with the device in any number of different ways
— for instance, by changing the shape and thickness of the rings,
their number or how far apart they are, or the patterns of copper
on the rings. They can make the original item vanish and make any
number of ghost images appear in its stead, as well as engineer
the appearance of those illusions.

In addition, although this prototype only presents one disguise,
future devices might be able to change the disguise at will. The
researchers are investigating whether placing antennas in the
ghosting area might tune what illusions are displayed.

So far this device only works in two dimensions, on radar scans
directed straight against its sides. The device could work in
three dimensions with concentric spheres, rather than the rings
used currently, Qiu noted.

In theory, ghost illusion devices that work in visible light can
be created as well. However, every feature of the device would
have to be shrunk to match visible light's smaller wavelengths.
For instance, to work against a wavelength of visible light of
about 600 nanometers or 600 billionths of a meter, one would need
copper loops only 50 nanometers wide, about 2,000 times thinner
than the average diameter of a human hair.

"The wavelength of light is not an issue, but fabrication is,"
Qiu said.

The scientists will detail their findings in a forthcoming issue
of the journal Advanced Functional Materials.